Emissions of mercaptan if not properly treated at the manufacturing site spreads into a blanket foul smell in ambient air and poses health hazards. Mercaptan is a heavy gas, wich once released, floats in the air and does not disperse easily with the smell lingering for hours. When inhaled, it is known to cause nausea, headache, irritation and vomiting, while prolonged exposure could be carcinogenic, affecting the lungs. Accordingly, it is of public concern that mercaptan be totally decomposed or converted into value added products to make the environment clean.
Sulfur compounds such as mercaptans, hydrogen sulfide etc are generated as effluents in a variety of industries, for example, in the paper, petroleum and gas refining, pharmaceutical, pesticide, and rubber manufacturing industries. The solutions adopted presently by the industry to control the emissions of the obnoxious gases like mercaptans are i) incineration of the pollutant gases (thermal combustion at higher temperatures or catalytic combustion), which has the draw back of producing again sulfur oxides that need scrubbing and also the cost of combustion is very high ii) adsorption of these gases on a solid phase like active charcoal, which has the draw back of vigorous heating-up and fire risks and also unable to recycle the charcoal iii) washing of the pollutant gases either with amines, sodium hydroxide or chlorine in the form of sodium hypochlorite or sodium chlorite solutions, which has the draw back of consumption of large amounts chemicals, make the treatment quite expensive. The chlorine and sodium hydroxide consumptions are high (8.5 and 7.6 mole/mole of methyl mercaptan removal respectively) iv) biodeoderization is achieved either by biomicroorganisms or by biofiltration which has the draw back of metabolization of coproducts and also requires installation of complex maintaining system. All of these solutions mentioned here have not only one or other drawbacks but also there is no recovery of any value added product obtained from the effluents.
Methanesulfonic acid is a high value product used as solvent, surfactant, catalyst in polymerization, alkylation and esterification reactions and also in electroplating industry. No method has been reported so far to our knowledge to obtain value-added product like methanesulfonic acid in-situ from an effluent gas methyl mercaptan generated during the chemical process in industries.
Some well-known methods of reactions of methyl mercaptan to produce methanesulfonic acid are mentioned herewith. For example, chlorination of methyl mercaptan produces methylsulfonyl chloride or methyl sulfonic acid that has the disadvantage of using corrosive chlorine gas as well as generation of large amounts of corrosive hydrogen chloride gas as byproduct that requires treatment. It is also known that mercaptans react with hydrogen peroxide in an alkaline medium to produce methanesulfonic acid via intermediate dialkyl disulfide. However, the peroxide to pollutant ratio of 5:1 is required to control odors as well as to get corresponding sulfonic acids. Here large excess of hydrogen peroxide is being used than stoichiometric quantities.
Reference is made to U.S. Pat. No. 5,605,635 (Feb. 25, 1997) wherein a process for purifying gaseous or liquid effluents containing sulfur-containing derivatives by basification of the effluent to be treated at pH>9 was described. Reference is also made to U.S. Pat. No. 4,729,835 (Mar. 8, 1988) wherein a process for waste treatment of dimethyl disulphide was disclosed using hydrogen peroxide at a pH 8.5 to 11.5 in the presence of tungstate catalyst. The drawbacks in the above processes are that there is no mention of recovering of any value-added product except to control the emission of the foul smelled gases.
Reference is further made to U.S. Pat. No. 4,239,696 (Dec. 16, 1980) wherein alkylsulfonic acids are prepared by oxidation of alkyl mercaptan or dialkyl disulfide with hydrogen peroxide. This is a synthetic method of preparation of alkylsulfonic acids and this work was not directed for the elimination of the obnoxious gases of effluent streams generated at chemical industries.